How to remove all pie menus in Blender 2.8?
There is no way to completely remove pie menus (without building your own version) but you can get closer.

Start by disabling Tab for Pie Menu. That will get you back the familiar tab usage.

Next I'd recommend enabling Pie Menu On Drag. That setting makes some buttons (like Tab, ` and others do double duty. Simply pressing them invokes a normal event, but pressing them and moving the mouse makes a pie menu popup).

Now start changing the keybinding in the keymap here is ShiftS Snapping. Set the identifier to "wm.call_menu" and the name to "VIEW3D_MT_snap". Old snapping menu is back.